1. Field of the Invention
The present invention relates to a heat-resistant metallic monolith suitable for use as a catalyst carrier, a heater or a catalytic converter and a method for manufacturing such a heat-resistant metallic monolith.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as catalysts or carriers for catalysts for removing, for example, nitrogen oxides, carbon monoxide and hydrocarbons present in the exhaust gas of internal combustion engines, such as automobiles, or filters for removing fine particles.
Whereas porous ceramic honeycomb structures continue to be a popular and useful material in such environments, there has been a desire to develop materials exhibiting greater mechanical strength and thermal resistance in hostile environments. Accordingly, a honeycomb structure produced by winding a metal plate in a corrugated fashion has been proposed in U.K. Patent 1492929 and used.
However, in this foil type metal honeycomb structure, the catalyst layers cannot be closely adhered to the metal substrate with a coating formed thereon because of its low porosity, and a ceramic catalyst readily peels off the metal substrate due to a difference in the thermal expansion between the ceramic catalyst and the metal substrate. Furthermore, telescope phenomenon readily occurs during the run cycle in which a metal to metal joint breaks and the metal substrate deforms in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil type metal honeycombs, yield of the rolling process is low, inviting high production cost.
Honeycomb structures manufactured by forming metal powders and by sintering the formed body are also known. Such honeycomb structures have been proposed in, for example, U.S. Pat. No. 4,758,272, Japanese Patent Laid-Open Nos. 57803/1982 and 57904/1982 and Japanese Patent Publication No. 6974/1982.
The honeycomb structure disclosed in U.S. Pat. No. 4,758,272 has a composition essentially consisting, as analyzed in weight percent, of 5 to 50% Al, 30 to 90% Fe, 0 to 10% Sn, 0 to 10% Cu, 0 to 10% Cr and not more than 1% Mg and/or Ca. This honeycomb structure has a porosity of about 25 to 75% by volume and a predetermined cell density.
However, since this honeycomb structure has no heat-resistant metal oxide coating formed thereon and is therefore not heat-resistant, an oxide film may be partially formed on the honeycomb structure when it is used as a filter, differentiating the characteristics, such as thermal expansion or ductility, and increasing the possibility of the honeycomb structure being broken.
The honeycomb structure disclosed in Japanese Patent Publication No. 6974/1982 is manufactured by binding a large number of small honeycomb components made of powders of catalytic activating substances which act as catalysts for use in automobile exhaust emission control, such as nickel, copper and chromium. However, oxidation process is performed at a low temperature to achieve catalytic activity, and it is therefore impossible to accomplish sufficient heat-resistance. Furthermore, this honeycomb structure is not a monolithic body and may be broken due to vibrations given to it during the operation.
The metal honeycomb structures disclosed in Japanese Patent Laid-Open Nos. 57803/1982 and 57804/1982 are manufactured by preparing mixture of metal powders, a thermosetting binder, colloidal silica and so on, by forming the mixture into a honeycomb configuration by extrusion and then sintering the shaped body after it has been hardened. Like U.S. Pat. No. 4,758,272, however, Japanese Patent Laid-Open Nos. 57803/1982 and 57804/1982 do not disclose the provision of a heat-resistant coating. Therefore, these metal honeycomb structures may be broken when used as, for example, a catalyst carrier.